Superstrate solar cell

ABSTRACT

A method of fabricating a solar cell includes forming a front contact layer over a substrate, and the front contact layer is optically transparent at specified wavelengths and electrically conductive. A first scribed area is scribed through the front contact layer to expose a portion of the substrate. A buffer layer doped with an n-type dopant is formed over the front contact layer and the first scribed area. An absorber layer doped with a p-type dopant is formed over the buffer layer. A back contact layer that is electrically conductive is formed over the absorber layer.

This application is a continuation of U.S. patent application Ser. No.13/207,058, filed Aug. 10, 2011, which is expressly incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to a solar cell and, moreparticularly, to a superstrate solar cell.

BACKGROUND

A solar cell includes a p-type doped absorber layer and an n-type dopedbuffer layer. For some superstrate solar cells, the absorber layer isdeposited at a high temperature after the buffer layer (e.g., CdS) isformed. However, there is a cross-diffusion of elements between thebuffer layer and the absorber layer during the deposition of theabsorber layer at a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exemplary superstrate solar cellmodule according to some embodiments;

FIGS. 2A-2H are schematic diagrams of the exemplary superstrate solarcell of FIG. 1 at various fabrication steps according to someembodiments; and

FIG. 3 is a flowchart of a method of fabricating the exemplarysuperstrate solar cell in FIG. 1 according to some embodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use, and do notlimit the scope of the disclosure.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,”“top,” “bottom,” etc. as well as derivatives thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of thepresent disclosure of one features relationship to another feature. Thespatially relative terms are intended to cover different orientations ofthe device including the features.

FIG. 1 is a schematic diagram of an exemplary superstrate solar cellmodule according to some embodiments. The superstrate solar cell module100 includes a substrate 102 (e.g., glass), a front contact layer 104(e.g., transparent conductive oxide, TCO), a buffer layer 106 (e.g.,CdS), an absorber layer 108 (e.g., copper indium gallium selenide,CIGS), and a back contact layer 110 (e.g., Mo). A positive node 120 iscoupled to the front contact layer 104 and a negative node 122 iscoupled to the back contact layer 110 for electrical connections.

The substrate 102 allows light at specified wavelengths to pass throughto generate electricity by the solar cell 100. In some embodiments, thesubstrate 102 comprises glass (e.g., soda-lime glass), flexiblepolyimide, or any other suitable material and has a thickness of about0.1-3 mm in some embodiments.

The first scribed area 112 vertically divides the front contact layer104. The first scribed area 112 is made by a scribing process (e.g.,mechanical scribing or laser scribing) to define an active area 118 andan interconnection area 119. The first scribed area 112 is filled by thebuffer layer 106 and the absorber layer 108. The second scribed area 114vertically divides the buffer layer 106 and the absorber layer 108. Thesecond scribed area 114 is made by a scribing process (e.g., mechanicalscribing or laser scribing) to define an electrical path for theinterconnection between the front contact layer 104 and the back contactlayer 110. The second scribed area 114 is filled by the back contactlayer 110.

The third scribed area 116 vertically divides the buffer layer 106, theabsorber layer 108, and the back contact layer 110. The third scribedarea 116 is made by a scribing process (e.g., mechanical scribing orlaser scribing) to provide isolation between adjacent cells. In someembodiments, the first and second scribed areas 112 and 114 have a widthof about 45-85 μm, and the third scribed area 116 has a width of about48-68 μm. The scribing process facilitates the fabrication flow ofsuperstrate solar cells.

The front contact layer 104 is a thin film which is opticallytransparent at specified (target) wavelengths and electricallyconductive. In some embodiments, the front contact layer 104 comprisesTCOs such as SnO₂, In₂O₃:Sn (ITO), In₂O₂:Ga, In₂O₃:F, Cd₂SnO₄ (CTO),Zn₂SnO₄, fluorine doped tin oxide (FTO), zinc oxide (ZnO) doped withgroup III elements such as aluminium-doped zinc oxide (ZnO:Al, AZO), orindium-doped cadmium oxide. Narrow lined metal grids (Ni—Al) can bedeposited on top of the TCO in order to reduce the series resistance insome embodiments.

The TCOs has a thickness of about 0.25-1.5 μm in some embodiments. TheTCOs are doped with n-type dopants, but some TCOs may be doped withp-type dopants. The front contact layer 104 can also comprise organicfilms using carbon nanotube networks and/or graphene, which can befabricated to be highly transparent to the infrared light, along withnetworks of polymers such as poly(3,4-ethylenedioxythiophene) or itsderivatives.

The buffer layer 106 can comprise CdS, In_(x)Se_(y), In(OH)_(x)S_(y),ZnO, ZnSe, ZnS, ZnS(O,OH), ZnIn₂Se₄, ZnMgO, any combination thereof, orany other suitable material, and doped with n-type dopants. The bufferlayer 106 has a thickness of about 0.01-0.1 μm in some embodiments.

The buffer layer 106 (e.g., CdS layer) helps the band alignment of thesolar cell device, builds a depletion layer to reduce tunneling, andestablishes a higher contact potential that allows higher open circuitvoltage (Voc) value. In some embodiments, the buffer layer 106 comprisesmultiple layers, e.g., a CdS bilayer consisting of a thinnerhigh-resistive layer, prepared either by evaporation or CBD, and athicker low-resistivity layer, doped with 2% In or Ga.

The absorber layer 108 is a p-type layer, and can comprise combinationsof Group-(I, III, VI) elements in the periodic table such as (Cu, Ag,Au|Al, Ga, In|S, Se, Te) including CIGS, or any other suitable material.I-III-VI₂ semiconductors, such as copper indium selenide (CIS) or CIGSare referred to as chalcopyrites because of their crystal structure.These materials are prepared in a wide range of compositions. For thepreparation of solar cells, slightly Cu-deficient compositions of p-typeconductivity can be used as the absorber layer 108.

In some embodiments, the absorber layer 108 comprises CuInSe₂, CuGaTe₂,Cu₂Ga₄Te₇, CuInTe₂, CuInGaSe₂, CuInGaSeS₂, CuInAlSe₂, CuInAlSeS₂,CuGaSe₂, CuAlSnSe₄, ZnIn₂Te₄, CdGeP₂, ZnSnP₂, any combination thereof,or any other suitable material. The absorber layer 108 has a thicknessof about 0.1-4 μm in some embodiments. The absorber layer 108 maycomprise multiple layers with different level of doping with p-typedopants dopant in some embodiments.

The back contact layer 110 that is electrically conductive can compriseMo, Pt, Au, Cu, Cr, Al, Ca, Ag, any combination thereof, or any othersuitable material, and has a thickness of about 0.5-1.5 μm in someembodiments. For example, Mo (molybdenum) as the back contact layer 110exhibits relatively good stability during processing, resistance toalloying with Cu and In, and low contact resistance to absorber layer108. The typical value of resistivity of Mo is about 5×10⁻⁵ Ωcm or less.

The superstrate solar cell 100 can be used in a tandem structure or amulti junction solar cell structure, in which the superstrate solar cellfunctions as a top cell for the shorter wavelength part of a lightsource. The glass substrate of the superstrate solar cell not only actsas a support but also as a part of the encapsulation, thus loweringmodule cost compared to a substrate solar cell.

Compared to the superstrate solar cell 100, the encapsulation of thesubstrate solar cell requires an additional glass to protect thestructure against environmental conditions and physical impacts, and atransparent/UV-resistant encapsulant or pottant, such as ethylene vinylacetate (EVA) is used. The conventional EVA pottants are subjected toyellow-to-brown discoloration upon photochemical or photothermaldegradation with regard to long-term weathering stability. Thesuperstrate solar cell 100 in FIG. 1 does not need the additionalencapsulation, incurring less EVA encapsulation package reliabilityproblems, thus enables a lower cost fabrication.

FIGS. 2A-2H are schematic diagrams of the superstrate solar cell in FIG.1 in various fabrication steps according to some embodiments. In FIG.2A, the substrate 102 is shown before adding other layers. In FIG. 2B,the front contact layer 104 is formed over the substrate 102. In oneexample, an Al-doped ZnO is RF-sputtered to form the front contact layer104. In another example, a combination of intrinsic zinc oxide (i-ZnO)and AZO are formed for the front contact layer 104 by RF magnetronsputtering and electrodeposition.

In FIG. 2C, the first scribed area 112 (an opening at this step) isscribed through the front contact layer 104, e.g., by using a laserscribing or mechanical scribing process. The first scribed area 112exposes a portion of the substrate 102 and vertically divides the frontcontact layer 104. The first scribed area 112 defines the active area118 and the interconnection area 119 in FIG. 1. (The first scribed area112 is later filled by the buffer layer 106 and the absorber layer 108in FIG. 2D and FIG. 2E.)

In FIG. 2D, the buffer layer 106 is formed over the front contact layer104 and the first scribed area 112 of the substrate 102. The bufferlayer 106 can be deposited by Chemical Bath Deposition (CBD), MolecularBeam Epitaxy (MBE), Metalorganic Chemical Vapor Deposition (MOCVD),Atomic Layer Deposition (ALD), Atomic Layer CVD (ALCVD), Physical VaporDeposition (PVD), evaporation, sputtering, Ion Layer Gas Reaction(ILGAR), Ultrasonic Spray (US), or any other suitable process.

In FIG. 2E, the absorber layer 108 is formed over the buffer layer 106and the first scribed area 112. Various thin-film deposition methods canbe used to deposit the absorber layer 108, such as Cu(In,Ga)Se₂. In onefabrication method, a two-step process is used that separates thedelivery of the metals from the reaction to form device quality filmsfor the absorber layer 108. The first step is the depositions ofprecursor materials, e.g., by sputtering using binary (Cu—Ga, Cu—Se,Ga—Se, In—Se), ternary (Cu—In—Ga, Cu—In—Al, Cu—In—Se) or quaternary(Cu—In—Ga—Se, Cu—Al—In—Se) targets.

The second step is selenization, e.g., by evaporating or sputtering Se,and thermal annealing in a controlled reactive or inert atmosphere(e.g., Ar or N₂) for optimum compound CIGS formation via thechalcogenization reaction (i.e., selenization of stacked metal orprecursor alloy layers). The selenization can be included in the firststep and the second step can be thermal annealing in some embodiments.In one exemplary process for the second step, a rapid thermal processing(RTP) is applied, which provides the conversion reaction of theprecursor layers to the chalcopyrite-semiconductor. In some embodimentsthe RTP process has ramp rates greater than 10° C./sec in Ar or N₂ andthen maintains at target temperatures of about 400-600° C. for about5-30 min or less.

The RTP process reduces the inter-diffusion problem between the absorberlayer 108 and the buffer layer 106 (e.g., CdS) interface due to shorterheating and cooling times. The shorter heating and cooling times alsofacilitates a high throughput/yield fabrication process due to fasterprocessing time. In another exemplary process for the second step, theprecursor films are reacted in H₂Se or Se vapor at about 400-500° C. forabout 30-60 min to form the absorber layer 108.

The precursor metals and/or alloys can be deposited by a variety ofmethods which involve vacuum or no vacuum. For example, sputtering,thermal evaporation, plasma-enhanced CVD, Atmospheric Pressure Metalorganic Chemical Vapor Deposition (AP-MOCVD), or flash evaporation. Inone vacuum-based approach, Cu/In layers are sputtered and reacted inhydrogen sulfide to form CuInS₂.

In another exemplary method of forming the absorber layer 108 is vacuumco-evaporation in which all the constituents, Cu, In, Ga, and/or Se, canbe simultaneously delivered to a substrate heated at 400-600° C. and theCu(In,Ga)Se2 film is formed in a single growth process.

In FIG. 2F, the second scribed area 114 (an opening at this step) isscribed through the absorber layer 108 and the buffer layer 106, e.g.,by using a laser scribing or mechanical scribing process. The secondscribed area 114 exposes a portion of the front contact layer 104 andvertically divides the buffer layer 106 and the absorber layer 108. Thesecond scribed area 114 defines an electrical path for theinterconnection between the front contact layer 104 and the back contactlayer 110 in FIG. 2G. The second scribed area 114 is later filled by theback contact layer 110 in FIG. 2G.

In FIG. 2G, the back contact layer 110 is formed over the absorber layer108 and the second scribed area 114 of the front contact layer 104. Theback contact layer 110 can be deposited, e.g., by e-beam evaporation,sputtering, or any other suitable method. In other embodiments, the backcontact layer 110 can comprise multiple layers, e.g., Na-doped Mo/Mobilayer.

In FIG. 2H, the third scribed area 116 (an opening) is scribed throughthe back contact layer 110, the absorber layer 108, and the buffer layer106, e.g., by using a laser scribing or mechanical scribing process. Thethird scribed area 116 exposes a portion of the front contact layer 104and vertically divides the buffer layer 106, the absorber layer 108, andthe back contact layer 110. The third scribed area 116 providesisolation between adjacent cells.

FIG. 3 is a flowchart of a method of fabricating the exemplarysuperstrate solar cell in FIG. 1 according to some embodiments. Detailssuch as exemplary processes and materials are as described above, andare not repeated again.

At step 302, a front contact layer is formed over a substrate, whereinthe front contact layer is optically transparent at specifiedwavelengths and electrically conductive. At step 304, a first scribedarea through the front contact layer is scribed to expose at least aportion of the substrate. At step 306, a buffer layer doped with ann-type dopant is formed over the front contact layer and the firstscribed area. At step 308, an absorber layer doped with a p-type dopantis formed over the buffer layer. At step 310, a back contact layer thatis electrically conductive is formed over the absorber layer.

According to some embodiments, a method of fabricating a solar cellincludes forming a front contact layer over a substrate, wherein thefront contact layer is optically transparent at specified wavelengthsand electrically conductive. A first scribed area is scribed through thefront contact layer to expose at least a portion of the substrate. Abuffer layer doped with an n-type dopant is formed over the frontcontact layer and the first scribed area. An absorber layer doped with ap-type dopant is formed over the buffer layer. A back contact layer thatis electrically conductive is formed over the absorber layer.

According to some embodiments, a solar cell includes a substrate and afront contact layer disposed over the substrate. A buffer layer dopedwith an n-type dopant is disposed over the front contact layer. Anabsorber layer doped with a p-type dopant is disposed over the bufferlayer. A back contact layer is electrically conductive and disposed overthe absorber layer. A first scribed area vertically divides the frontcontact layer. The first scribed area is filled in by the buffer layerand the absorber layer. The front contact layer is optically transparentat specified wavelengths and electrically conductive.

A skilled person in the art will appreciate that there can be manyembodiment variations of this disclosure. Although the embodiments andtheir features have been described in detail, it should be understoodthat various changes, substitutions and alterations can be made hereinwithout departing from the spirit and scope of the embodiments.Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosed embodiments, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure.

The above method embodiment shows exemplary steps, but they are notnecessarily required to be performed in the order shown. Steps may beadded, replaced, changed order, and/or eliminated as appropriate, inaccordance with the spirit and scope of embodiment of the disclosure.Embodiments that combine different claims and/or different embodimentsare within the scope of the disclosure and will be apparent to thoseskilled in the art after reviewing this disclosure.

What is claimed is:
 1. A method of fabricating a solar cell, comprising:forming a front contact layer over a substrate wherein the front contactlayer is optically transparent; scribing a first scribed area throughthe front contact layer; forming a buffer layer over the front contactlayer and the first scribed area; forming an absorber layer over thebuffer layer; and forming a back contact layer over the absorber layer.2. The method of claim 1, wherein the first scribed area is formed bymechanical scribing or laser scribing.
 3. The method of claim 1, furthercomprising scribing a second scribed area through the absorber layer andthe buffer layer to expose a portion of the front contact layer.
 4. Themethod of claim 3, wherein the second scribed area is filled with theback contact layer.
 5. The method of claim 4, further comprisingscribing a third scribed area through the back contact layer, theabsorber layer, and the buffer layer to expose a portion of the frontcontact layer.
 6. The method of claim 1, wherein forming the absorberlayer comprises: depositing precursor metal layers; and applying a rapidthermal processing to the precursor metal layer.
 7. The method of claim6, wherein forming the absorber further comprises evaporating elementalSe.
 8. The method of claim 6, wherein the rapid thermal processing isperformed at a temperature from about 400 to 600° C. for about 30minutes or less.
 9. The method of claim 6, wherein the precursor metallayers include Cu, In, Ga, Se, or any combination thereof.
 10. A solarcell, comprising: a substrate; a front contact layer disposed over thesubstrate; a buffer layer disposed over the front contact layer; anabsorber layer disposed over the buffer layer; a back contact layerdisposed over the absorber layer; a first scribed area that verticallydivides the front contact layer, wherein the front contact layer isoptically transparent.
 11. The solar cell of claim 10, wherein the firstscribed area is filled in by the buffer layer and the absorber layer.12. The solar cell of claim 10, further comprising a second scribed areathat vertically divides the buffer layer and the absorber layer, whereinthe second scribed area is filled by the back contact layer.
 13. Thesolar cell of claim 12, further comprising a third scribed area thatvertically divides the buffer layer, the absorber layer, and the backcontact layer.
 14. The solar cell of claim 10, wherein substratecomprises glass or polyimide.
 15. The solar cell of claim 10, whereinthe front contact layer comprises transparent conductive oxide (TCO).16. The solar cell of claim 10, wherein the buffer layer comprises CdS,In_(x)Se_(y), In(OH)_(x)S_(y), ZnO, ZnSe, ZnS, ZnS(O,OH), ZnIn₂Se₄,ZnMgO, or any combination thereof.
 17. The solar cell of claim 10,wherein the absorber layer comprises CuInSe₂, CuGaTe₂, Cu₂Ga₄Te₇,CuInTe₂, CuInGaSe₂, CuInGaSeS₂, CuInAlSe₂, CuInAlSeS₂, CuGaSe₂,CuAlSnSe₄, ZnIn₂Te₄, CdGeP₂, ZnSnP₂, or any combination thereof.
 18. Thesolar cell of claim 10, further comprising: a second scribed area thatvertically divides the buffer layer and the absorber layer, wherein thesecond scribed area is filled by the back contact layer; and a thirdscribed area that vertically divides the buffer layer, the absorberlayer, and the back contact layer, wherein: the first scribed area isfilled in by the buffer layer and the absorber layer; the front contactlayer comprises transparent conductive oxide (TCO); the buffer layercomprises CdS, In_(x)Se_(y), In(OH)_(x)S_(y), ZnO, ZnSe, ZnS, ZnS(O,OH),ZnIn₂Se₄, ZnMgO, or any combination thereof; the absorber layercomprises CuInSe₂, CuGaTe₂, Cu₂Ga₄Te7, CuInTe₂, CuInGaSe₂; CuInGaSeS₂,CuInAlSe₂, CuInAlSeS₂, CuGaSe₂, CuAlSnSe₄, ZnIn₂Te₄, CdGeP₂, ZnSnP₂, orany combination thereof; the back contact layer comprises Mo, Pt, Au,Cu, Cr, Al, Ca, Ag, or any combination thereof; and the substratecomprises glass or polyimide.
 19. A method of fabricating a solar cell,comprising: forming a front contact layer over a substrate wherein thefront contact layer is optically transparent; scribing a first scribedarea through the front contact layer; forming a buffer layer over thefront contact layer and the first scribed area; forming an absorberlayer over the buffer layer; scribing a second scribed area through theabsorber layer and the buffer layer; forming a back contact layer overthe absorber layer and the second scribed area; and scribing a thirdscribed area through the back contact layer, the absorber layer, and thebuffer layer.
 20. The method of claim 19, wherein forming the absorberlayer comprises: evaporating Se; depositing precursor metal layersincluding Cu, In, Ga, Se, or any combination thereof over the bufferlayer; and applying a rapid thermal processing to the precursor metallayers.